1. Field of the Invention
The present invention relates to a control system and control method for controlling an information rate in a mobile communication system and a radio network controller preferably used for the control system and control method.
Particularly, the present invention relates to a control system and control method for controlling the information rate of downlink in a mobile packet communication system and a radio network controller preferably used for the control system and control method.
2. Description of the Related Art
A conventional mobile packet communication system addresses time-division multiplexed data signals to a plurality of users (mobile stations) on a single radio channel (hereafter referred to as a channel) and transmits the data signals to the users from a viewpoint of the efficiency of frequency utilization, delay tolerance in packet communication, and the saving of code resources in downlink.
FIG. 1 shows a channel configuration for a conventional packet communication system specified in 3GPP (Third Generation Partnership Project).
The channel configuration shown in FIG. 1 uses a downlink shared channel (DSCH) for time-division multiplexing and transmitting data signals addressed to mobile stations 401 and 402 through downlink.
The channel configuration shown in FIG. 1 uses an associated dedicated physical channel (A-DPCH) for transmitting control signals addressed to the mobile stations 401 and 402 through downlink.
Moreover, the channel configuration shown in FIG. 1 uses a dedicated physical channel (DPCH) for transmitting both data signals and control signals addressed to the mobile stations 401 and 402 in uplink because the possibility of exhaustion of code resources is very low in uplink and the necessity, for using a shared channel such as a DSCH is low.
In this case, A-DPCH-1 and A-DPCH-2 are dedicated channels for a base station 30 to transmit a control signal to the mobile stations 401 and 402 respectively, which are always transmitted through code division multiplexing using codes different for each of the mobile stations 401 and 402.
Moreover, the DSCH is a shared channel for the base station 30 to transmit data signals to the mobile stations 401 and 402 and is always transmitted through code-division multiplexing for changing the transmission timing for each of the mobile stations 401 and 402 by using the same code.
In this case, to transmit data signals from the network (base station 30) to the mobile station 401 or 402 by using a DSCH, the message “a DSCH is transmitted” is previously notified to the mobile station 401 or 402 through the radio frame of an A-DPCH before the radio frame of the DSCH.
Then, the mobile station 401 or 402 receiving the above notice starts receiving the DSCH only when it is determined that the DSCH is coming.
By using the above system, the base station 30 can freely change the mobile stations 401 and 402 for transmitting data signals by using the DSCH.
In FIG. 1, the A-DPCH-1 and A-DPCH-2 and the DSCH are set as downlinks. However, the DSCH is not always set but it is set only when the above notice is received through the A-DPCH-1 or A-DPCH-2.
Because the DSCH is discretely transmitted on a time basis, it is impossible to apply closed-loop transmission power control to the DSCH. Therefore, closed-loop transmission power control is applied to the temporally-continuous A-DPCH or DPCH and the transmission power of the DSCH is controlled so as to interlock with the transmission power of the A-DPCH with a predetermined offset which is previously decided by a telecommunication operator.
For example, when the transmission power of the A-DPCH-1 addressed to the mobile station 401 is 20 dBm at the time t and the DSCH is transmitted to the mobile station 401 at the time t, the transmission power of the DSCH addressed to the mobile station 401 at the time t becomes 30 dBm if the above offset is equal to 10 dBm.
Moreover, for example, when the transmission power of the A-DPCH-2 addressed to the mobile station 402 is 22 dBm at the time t+1, the transmission power of the DSCH addressed to the mobile station 402 at the time t+1 similarly becomes 32 dBm.
Furthermore, in the case of a conventional mobile packet system specified in 3GPP, the information rate of the DSCH can be changed at every transmission time interval (TTI) at each of mobile stations 401 and 402.
However, it is unknown what can be used as a trigger to change an information rate of the DSCH at present, and an information rate set at start of communication is not changed after the start of the communication.
In general, it is a problem for a mobile packet communication system to maintain communication quality, decrease transmission delay (increase an information rate), and simultaneously connect with numerous mobile stations 40 (house more users). In this case, it is known that an information rate is proportional to a transmission power.
That is, it is known that when a higher information rate is necessary, a larger transmission power is necessary and when communication is performed at a low information rate, only a small transmission power is necessary.
In the case of a conventional mobile packet communication system, however, because the information rate of the DSCH is a fixed information rate and the A-DPCH is a dedicated channel for code-division-multiplexing a control signal addressed to each mobile station 40, a system capacity is pressured due to the increase in the transmission power or the number of code resources consumed by the A-DPCH when the number of mobile stations 40 simultaneously connected increases.
Therefore, a problem occurs in that it is difficult to maintain communication quality and decrease transmission delay.
To solve the above problem, a mechanism is known which maintains communication quality of the whole mobile packet communication system and decreases transmission delay by observing the transmission power and the number of code resources consumed by the A-DPCH when admitting a connection request from a new mobile station 40.
The above mechanism does not admit the connection request from the new mobile station 40 when either of the transmission power and the number of code resources is almost exhausted.
However, the above mechanism has a problem that it cannot connect with numerous mobile stations 40 (cannot house more users) at the same time though it is preferable not to reject connection requests from new mobile stations 40 as far as possible.